Many organic polymeric materials, including polyphenylene ethers, undergo some mode of degradation when exposed to the high energy photons of ultraviolet radiation. The degradation manifests itself, depending on the polymer materials, in yellowing, discoloration, embrittlement and other loss of physical properties. For example, noticeable color changes occur in unstabilized formulations of polyphenylene ethers due to formation of yellow color bodies by a complex photodegradation reaction.
The use of ultraviolet radiation absorbers with various resins such as polyesters, polyolefins, vinyls and polystyrenes to provide protection against attack by ultraviolet radiation is known in the art. The ultraviolet radiation absorber (UV absorber) functions by reason of its ability to screen out the damaging ultraviolet light portions of light due to its very high absorptivity relative to that of the polymer. In order to qualify as a successful ultraviolet light absorber for a thermoplastic resin, there are several requirements which the absorber must fulfill. The absorber must have a high specific absorptivity in the range of wavelengths that are deleterious to the thermoplastic resin, the UV absorber must be compatible with the thermoplastic resin to be stabilized and the absorber must have a sufficiently low volatility to permit its continued residence in the thermoplastic resin. It is also preferable that the absorber not significantly absorb in the visible regions of the spectrum or color will be imparted to the thermoplastic to which it is added.
Various UV absorbers have been used in combination with thermoplastic resins to reduce the effect of coloring induced by UV radiation with varying success. Common UV absorbers include benzophenones and benzophenone derivatives, benzotriazoles and benzotriazole derivatives, benzoate esters, phenyl salicylates, derivatives of crotonic acid, malonic acid esters and cyanoacrylates. These compounds are described more particularly by Olson et al. in U.S. Pat. No. 4,322,455, assigned to the same assignee as the present invention and incorporated herein by reference.
It has been discovered that the benzofurans (described more particularly below) function as UV absorbers in polyphenylene ether resins under conditions of mild exposure to UV radiation, such as that experienced indoors under conventional fluorescent lamps. They can be incorporated into the resin by bulk addition or may be applied to a substrate within a coating material. Surprisingly, these compounds are very photo-oxidatively stable, have high thermal stability and are compatible with polyphenylene ethers. These compounds also exhibit the necessary high absorptivity and low volativity for UV absorbers.
Lang and Lawuet have utilized related compounds, i.e., methoxy-substituted 2-arylbenzofurans, as sunscreen agents in cosmetics, as described in German Pat. No. 2,543,099. The invention herein marks the first use of arylbenzofurans as UV stabilizers in a solid thermoplastic resin.